Endovascular stents have become increasingly important in medical procedures to restore the function of bodily lumens. With generally open tubular structures, the stents typically have apertured or lattice-like walls of a metallic or polymeric base, and can be either balloon expandable or self-expanding. A stent is typically deployed by mounting the stent on a balloon portion of a balloon catheter, positioning the stent in a body lumen, and expanding the stent by inflating the balloon. The balloon is then deflated and removed, leaving the stent in place. Stents help reduce the probability and degree of vessel blockage from restenosis.
An increasing number of stents for treating vascular conditions are being coated with protective materials and bioactive drugs. A variety of stent coatings and compositions have been proposed to provide localized therapeutic pharmacological agents and treatment of a vessel at the site being supported by the stent. Stent coatings with various families of drug polymer chemistries have been used to increase the effectiveness of stenting procedures and to control drug-elution properties. For example, polymeric coatings can be made from polyurethane, polyester, polylactic acid, polyamino acid, polyorthoester, and polyphosphate ester. Examples of drug or bioactive agents include antirestonotic and anti-inflammatory compounds.
Medical research indicates a greater effectiveness of vascular stents when stents are coated with pharmaceutical drugs that help prevent or treat medical conditions such as restenosis and thrombosis. These drugs may be released from a coating while in the body, delivering their patent effects at the site where they are most needed. The localized levels of the medications can be elevated, and are therefore potentially more effective than orally or intravenously delivered drugs. Furthermore, drugs released from tailored stent coatings can have controlled, timed-release qualities, eluting their bioactive agents over hours, weeks or even months. Stent coatings typically have a drug or active agent, which has been dissolved or dispersed throughout the polymeric material and physically constrained within the polymer. The sustained release of drugs generally relies upon either degradation of the polymer or diffusion through the polymer to control the elution of the compounds.
Stents can be coated with a polymer or combination of a polymer and a pharmaceutical agent or drug by application techniques such as dipping, spraying, painting, and brushing. In many of the current medical device or stent coating methods, a composition of a drug and a polymer in a solvent is applied to a device to form a substantially uniform layer of drug and polymer. The concentration of the pharmaceutical agent or drug applied to the stent varies depending on the pharmaceutical agent or drug and the intended use of the pharmaceutical agent or drug. Generally, the dose of pharmaceutical agent or drug coated on a stent ranges from nanograms to milligrams. A problem arises when trying to coat stents with lower doses of pharmaceutical agent or drug.
A common solvent for the polymers and drugs employed is usually required, and techniques have been developed to micronize the drugs into small particles so that the drugs can be suspended in the polymer solution. Micronization can be time consuming, and may result in a degradation or loss of desired therapeutic properties of the drug. A method of using micronized drugs and layering a drug-coated stent using pharmacological and polymeric agents is described by Guruwaiya et al. in U.S. Pat. No. 6,251,136 issued Jun. 26, 2001. A pharmacological agent is applied to a stent in dry, micronized form over a sticky base coating. A membrane-forming polymer, selected for its ability to allow the diffusion of the pharmacological agent therethrough, is applied over the entire stent. More specifically, a stent, typically a metal stent, has a layer of a sticky material applied to selected surfaces of the stent. A pharmacological agent is layered on the sticky material and a membrane forming a polymer coating is applied over the pharmacological agent. The membrane is formed from a polymer that permits diffusion of the pharmacological agent over a predetermined time period.
A method of applying drug-release polymer coatings that uses solvents is described in “Method of Applying Drug-Release Coatings”, Ding et al., U.S. Pat. No. 5,980,972 issued Nov. 9, 1999. A polymer is dissolved in one solvent and a drug is dissolved or suspended in a similar or different type of solvent. The solutions are applied either sequentially or simultaneously onto the devices by spraying or dipping to form a substantially homogenous composite layer of the polymer and the biologically active material.
Many of the drug-coated stents in recent years have been sprayed with rather than dipped in a drug-polymer solution. Spray coating, a currently preferred method for coating stents, can result in a significant amount of spray material lost during the process and when expensive drugs are used in these coatings, the use of spray coating may be costly. Another drawback to spraying is that spraying deposits an inexact amount of therapeutic agent on the stent, potentially delivering more or less than desired. This is a problem where the therapy requires a more controlled amount of drug be administered to the target site. This is also a problem where a very small concentration of therapeutic agent is to be applied to and eluted from the stent.
Dip coating was used with early stents and other medical-device designs that were of relatively open construction fabricated from wires or from ribbons. Such coating methods were performed by manually dipping the stent in a liquid, and then removing the stent and drying it. The dipping process requires care to avoid excess liquid on the stent framework or inconsistent drying of the liquid, otherwise the apertures can become blocked unnecessarily. Applying a thick coating tends to exacerbate webbing and bridging problems, and increasing the solids content of the coating solution also increases webbing and bridging between the struts. Any coating method needs to avoid webbing, as well as control the weight and thickness of a coating.
Newer stents that are of less open construction, such as catheter-deployed, self-expanding stents are more difficult to coat evenly using a dipping method. Nevertheless, one advantage of dip coating is the ability to process a greater number of stents in a more efficient manufacturing process.
Dipping as a method of coating medical devices is described in U.S. Patent Application No. 20020082679 published Jun. 27, 2002 entitled “Delivery or Therapeutic Capable Agents” to Sirhan and Yan. Barry et al. describe another method of dip coating a stent with a polymer composition that can be used for delivering substantially water-insoluble drugs in “Loading and Release of Water-Insoluble Drugs”, U.S. Pat. No. 6,306,166 issued Oct. 23, 2001.
Multiple dips can be used to build up the weight and thickness of the coating, but each subsequent dip may affect the coating already deposited. A coating can re-dissolve in a second coating solution, causing some loss of the first layer of coating. Also, applications of multiple dip coats from low concentration solutions can have the effect of reaching a limiting loading level as equilibrium is reached between the solution concentration and the amount of coating with or without a pharmaceutical agent. One such method that applies a plurality of relatively thin coatings on an open-lattice stent is disclosed in “Drug Release Stent Coating”, Ding et al., U.S. Pat. No. 6,358,556 issued Mar. 19, 2002. The stents are coated by dipping or, preferably, spraying the stent with a solvent mixture of uncured polymeric silicone material with a cross-linker and a finely divided biologically active species.
Accordingly, what is needed is a method for coating medical devices such as stents with drug-polymer coating with more accuracy than current methods of coating and that overcomes the deficiencies and limitations described above.